1. Field of the Invention
The present invention relates to a technique suitable for a semiconductor device and a process for producing the same, in particular, a semiconductor device having a capacitor, for example, a dynamic random access memory (DRAM), and a process for producing the same.
2. Description of Related Art
It is known that for large scale integration of semiconductor devices or other purposes, a tantalum pentoxide film having a dielectric constant of several tens is adopted instead of a silicon oxide film (dielectric constant: about 4) or a silicon nitride film (dielectric constant: about 7), which has been conventionally used as a capacitor dielectric film (for example, JP-A No. 139288/1998).
As a process for producing a capacitor having the tantalum pentoxide dielectric film, there is known a process of forming a tantalum pentoxide film on a capacitor bottom electrode, heat-treating the film to be crystallized, and then forming a capacitor top electrode thereon. The reason why the tantalum pentoxide film is heat-treated is to use a characteristic of tantalum pentoxide that its dielectric constant becomes larger when it is crystallized, and obtain a capacitor having a large electric capacitance. However, it is known that in order to crystallize the tantalum pentoxide film sufficiently in this production process, it is necessary to conduct heat treatment at a high temperature of 750xc2x0 C. or more (for example, JP-A No. 12796/2000). The temperature at which the heat treatment for crystallizing a dielectric film is conducted is referred to as the xe2x80x9ccrystallization temperaturexe2x80x9d hereinafter.
Capacitor structure will be described before description on problems resulting from the matter that heat treatment at 750xc2x0 C. or more, which is a relatively high temperature, is required for crystallization.
A capacitor using a tantalum pentoxide film as a dielectric film is roughly classified into MIS (metal-insulator-semiconductor) structure, which uses a polycrystalline silicon film as a bottom electrode, and MIM (metal-insulator-metal) structure, which uses a metal film as a bottom electrode. Differences between the MIS structure and the MIM structure are the following: 1) Their bottom electrode materials are different. 2) A barrier metal is necessary for the MIM structure. The barrier metal is formed between the bottom electrode and a plug connected to the bottom electrode, and is necessary for preventing reaction between the bottom electrode and the plug. If the plug reacts with the bottom electrode, a bad effect is produced on electrical conductivity. An example of the barrier metal is titanium nitride formed between a plug made of polycrystalline silicon and a bottom electrode made of ruthenium.
The following will describe problems resulting from the matter that heat treatment at 750xc2x0 C. or more, which is a relatively high temperature, is required for crystallization in a process for forming a tantalum pentoxide insulator film. The MIS structure is heat-treated, thereby oxidizing silicon of its bottom electrode. As a result, the capacitance thereof drops. The reason for the drop is that since silicon is more easily subjected to thermodynamic oxidation than tantalum, silicon reduces the tantalum pentoxide film during heat treatment for crystallization of tantalum pentoxide so that a silicon oxide film, which has a small dielectric constant, is formed. The problem of the drop in the capacitor capacitance is also generated in the case of forming a silicon nitride film on the surface of the bottom electrode to prevent oxidation of silicon. Because the silicon nitride film is oxidized for the same reason so that the capacitor capacitance drops. In the MIM structure, its barrier metal is oxidized by oxygen diffusing in its metal electrode even if the metal electrode itself does not undergo any problem of oxidation. As a result, the electrical conductivity thereof is damaged. The reason for the damage is as follows: For example, in the case that the bottom electrode is made of ruthenium, oxygen atoms easily diffuse through ruthenium so that oxygen accumulates in the bottom electrode in the step in which the tantalum pentoxide film is formed; therefore, the barrier metal is oxidized by the accumulating oxygen in a subsequent step of heat treatment for crystallization of the dielectric film.
The respective problems peculiar to the MIS structure and the MIM structure do not depend on atmosphere at the time of the heat treatment for crystallization. When a capacitor is formed using a tantalum pentoxide film, heat treatment maybe conducted in oxygen atmosphere. Even if the oxidation temperature in the heat treatment is made low so that the oxidation of the bottom electrode and the barrier metal can be suppressed, the problems are not fundamentally solved if the crystallization temperature cannot be made low.
As far as tantalum pentoxide is used as the capacitor dielectric film, it is very difficult to make the temperature for crystallizing tantalum pentoxide as low as 750xc2x0 C. or less. Examples of the heat treatment in oxygen atmosphere include heat treatment performed in oxidation atmosphere to repair oxygen vacancy in the capacitor dielectric film, and heat treatment performed in oxidation atmosphere to remove residual carbon, which causes leakage current in the capacitor dielectric film formed by chemical vapor deposition (CVD) or the like.
Thus, in order to solve the problems based on a relatively high heat treatment temperature for crystallizing the tantalum pentoxide insulator film, the present inventors added niobium pentoxide to tantalum pentoxide and examined change in various properties.
First, FIG. 10 shows experimental results about a MIM structure. As a sample, there was used a film made of a composition tantalum pentoxide and niobium pentoxide and formed on a structure of Pt (200 nm)/Ti (10 nm)/SiO2 (100 nm) by sputtering. To form the film, a mixed gas of N2 and O2 (pressure ratio between N2 and O2:1/1) having a pressure of 10 mTorr was used. The substance temperature was 300xc2x0 C., and the film thickness was 20 nm. After the formation of the insulator film, heat treatment was conducted within the temperature range of 500 to 800xc2x0 C. in nitrogen gas flow for 1 minute. Thereafter, heat treatment was conducted at a temperature of 500xc2x0 C. in oxygen gas flow for 2 minutes. The temperatures for crystallizing a solid solution of tantalum pentoxide and niobium pentoxide formed under the above-mentioned conditions and the dielectric constants thereof after the crystallization were compared in the case that the ratio of Nb was 0%, 10%, 50%, 90% and 100%, respectively. The results are shown in FIG. 10. The transverse axis thereof represents the Nb content, and the vertical axes thereof represent the crystallization temperature and the dielectric constant. In the case that the Nb ratio was 0%, that is, in the case of the film made only of tantalum pentoxide, the crystallization temperature was about 750xc2x0 C. and the dielectric constant was about 30. As the Nb content was increased, the crystallization temperature lowered and simultaneously the dielectric constant increased. In the case that the ratio of Nb was 100%, that is, in the case of the film made only of niobium pentoxide, the crystallization temperature was about 500xc2x0 C. and the dielectric constant was about 60. In order to set the crystallization temperature to 700xc2x0 C. or less, at which oxidation of the bottom electrode and the barrier metal can be suppressed up to such a degree that no problem is caused, it is advisable that Nb is added at a ratio of at least 10%
Experimental results about a MIS structure are shown in FIG. 11. The temperatures for crystallizing a solid solution of tantalum pentoxide and niobium pentoxide formed on silicon and the dielectric constants thereof after the crystallization were compared in the case that the ratio of Nb was 0%, 10%, 50%, 90% and 100%, respectively. The results are shown in FIG. 11. The transverse axis thereof represents the Nb content, and the vertical axes thereof represent the crystallization temperature and the dielectric constant. In the case that the Nb ratio was 0%, that is, in the case of the film made only of tantalum pentoxide, the crystallization temperature was about 750xc2x0 C. and the dielectric constant was about 40. In order to set the crystallization temperature to 700xc2x0 C. or less, at which oxidation of the bottom electrode can be suppressed up to such a degree that no problem is caused, it is advisable that Nb is added at a ratio of 60% or more. As the Nb content was increased, the crystallization temperature lowered and simultaneously the dielectric constant increased. This tendency is the same as in FIG. 10. However, FIG. 11 is different from FIG. 10 in that at an Nb ratio of 50%, the crystallization temperature rises up to about 750xc2x0 C.
FIG. 12 shows results of comparison of leakage current densities of insulator films having different Nb ratios. The transverse axis represents voltage, and the vertical axis represents the leakage current density. The heat treatment temperature was 700xc2x0 C. As the Nb content was increased, the leakage current density increased.
As described above, in order to avoid a relatively high heat treatment temperature when a tantalum pentoxide insulator film is adopted as the dielectric film, it is effective to use a film to which niobium pentoxide is added or a film made only of niobium pentoxide. As understood from FIG. 12, however, the inventors found out a problem that when niobium pentoxide is added, leakage current density increases.
The present invention for solving the above-mentioned problems will be described hereinafter.
According to a first aspect of the present invention, an insulator film of a capacitor is made of a layered film composed of a niobium pentoxide film and a tantalum pentoxide film.
As is evident from FIGS. 10, 11 and 12, a tantalum pentoxide film has a small leakage current but has a high crystallization temperature. Contrarily, a niobium pentoxide film has a low crystallization temperature but has a large leakage current. On the basis of the results, the inventors have considered that a layered film of a tantalum pentoxide film and a niobium pentoxide film is effective. Specifically, a niobium pentoxide film is formed on a bottom electrode, and the resultant is heat-treated at a temperature lower than 750xc2x0 C. so as to be crystallized. A tantalum pentoxide film is formed thereon, and the resultant is heat-treated. In this way, tantalum pentoxide is laminated on the undercoat having a crystal structure of niobium pentoxide. As a result, the crystallization temperature of the tantalum pentoxide film is affected by the crystal structure of the niobium pentoxide film so as to be made low. Simultaneously, the tantalum pentoxide film is present in the layered film; therefore, the leakage current density can also be kept small.
To verify the effect of the layered film, a tantalum pentoxide single-layered film (Ta2O5), a bi-layered film composed of a tantalum pentoxide film and a niobium pentoxide film (Ta2O5/Nb2O5), and a niobium pentoxide single-layered film (Nb2O5) were formed, and then the crystallization temperatures and the leakage current densities thereof were compared. Each of the tantalum pentoxide single-layered film and the niobium pentoxide single-layered film was formed by forming a corresponding single-layered film having a film thickness of 20 nm, heat-treating the film at a temperature of 500 to 750xc2x0 C. in nitrogen for one minute, and then heat-treating the film at a temperature of 500xc2x0 C. in oxygen for two minutes. The layered film composed of tantalum pentoxide and niobium pentoxide was formed through the step of forming a niobium pentoxide film having a film thickness of 5 nm and then heat-treating the film at a temperature of 500xc2x0 C. in nitrogen for one minute and the step of forming a tantalum pentoxide film having a film thickness of 15 nm on the niobium pentoxide, heat-treating the layered film at a temperature of 500 to 750xc2x0 C. in nitrogen for one minute, and then heat-treating the layered film at a temperature of 500xc2x0 C. in oxygen for two minutes.
FIG. 13 shows the dependency of the dielectric constant of each of the above-mentioned dielectric films on heat treatment temperature. The transverse axis represents the heat treatment temperature and the vertical axis represents the dielectric constant. The tantalum pentoxide single-layered film is amorphous after the formation thereof, and the dielectric constant thereof is about 20. When the tantalum pentoxide single-layered film is heat-treated at 750xc2x0 C., the film is crystallized and the dielectric constant thereof increases to about 30. On the other hand, the niobium pentoxide single-layered film is amorphous after the formation thereof and the dielectric constant thereof is about 30. However, this film is crystallized by heat treatment at a temperature of at lowest 500xc2x0 C. so that the dielectric constant thereof increases to about 60. The layered film composed of tantalum pentoxide and niobium pentoxide, suggested by the present invention, is already crystallized immediately after the formation of the tantalum pentoxide film and the dielectric constant thereof is about 50. This results from the matter that the crystallization temperature of the tantalum pentoxide film is made lower since the tantalum pentoxide film is formed on the crystal structure of the niobium pentoxide film which is already crystallized. In general, the tantalum pentoxide film on the niobium pentoxide film is sufficiently crystallized even by heat treatment at such a temperature that tantalum pentoxide is not easily crystallized. The dielectric constant of the tantalum pentoxide film increases to substantially equal to that of the niobium pentoxide single-layered film.
Next, comparison between leakage current densities is shown in FIG. 14. The transverse axis thereof represents voltage, and the vertical axis thereof represents leakage current density. About the tantalum pentoxide single-layered film, the crystallization temperature is set to 750xc2x0 C. About the layered film composed of the tantalum pentoxide film and the niobium pentoxide film, and the niobium pentoxide single-layered film, the crystallization temperature is set to 500xc2x0 C. The leakage current of the niobium pentoxide film is by far larger than that of the tantalum pentoxide film. However, by laminating the tantalum pentoxide film thereon, the property substantially equal to that of the tantalum pentoxide single-layered film can be obtained. This demonstrates that the leakage current of the layered film composed of tantalum pentoxide and niobium pentoxide is reinforced by the tantalum pentoxide film.
In other words, by forming a lamination of a tantalum pentoxide film and a niobium pentoxide film, drawbacks of the respective films are cancelled so that the crystallization temperature of the tantalum pentoxide film is lowered to a temperature substantially equivalent to that of the niobium pentoxide single-layered film. A dielectric constant as high as that of the niobium pentoxide film can be obtained. Simultaneously, a capacitor having a leakage current density as low as that of the tantalum pentoxide single-layered film can be realized.
In the present context, examples wherein a layered film composed of a tantalum pentoxide film and a niobium pentoxide is used have been described. However, the present invention is not limited to the examples. The basic conception thereof is that: the formation of a dielectric film made of a material which originally has a high crystallization temperature on a dielectric film made of a material which has a low crystallization temperature causes the crystallization temperature of the upper layer to be lowered and causes the oxidation of the bottom electrode and the barrier metal to be prevented; and the upper layer which has a better leakage current property than the lower layer makes it possible to decrease leakage current between the bottom electrode and the top electrode. For example, both of the upper and lower layers may be made of a solid solution of tantalum pentoxide and niobium pentoxide. In this case, when the upper layer and the lower layer are represented by (Ta1xe2x88x92xNbx)2O5/(Ta1xe2x88x92yNby)2O5, it is effective to keep the relationship of x less than y. In short, it is advisable that the niobium pentoxide content in the lower layer is larger than the niobium pentoxide content in the upper layer. The lower layer satisfying this relationship is crystallized at a temperature lower than the upper layer and causes the crystallization temperature of the upper layer to be lowered. The upper layer having a better leakage current property than the lower layer makes it possible to decrease leakage current. However, as the difference between the niobium pentoxide content in the formed upper layer and the niobium pentoxide content in the lower layer is smaller, this effect is smaller.
According to a second aspect of the present invention, an insulator film of a capacitor is made of a layered film of niobium pentoxide films.
Specifically, a niobium pentoxide film is firstly on a bottom electrode, and then crystallized by heat treatment at a low temperature. A niobium pentoxide film is formed thereon and then heat-treated. The use of the niobium pentoxide films, which have a low crystallization temperature, as dielectric films, makes it possible to prevent the bottom electrode and the barrier metal from being oxidized. The formation of the dielectric film at plural stages causes the boundary of grains, which functions as a leakage pass, to be separated. Furthermore, to make the film thickness per layer of the layered film small causes residual carbon, which causes serious problems in heat treatment at low temperature, to be easily removed. Therefore, the leakage current of the capacitor can be decreased. Moreover, to make the film thickness per layer of the layered film small also causes a decrease in stress against the film and improvement in the quality of the film and the morphology thereof, thereby contributing to the decrease in the leakage current.
To demonstrate the effect of the formation of the niobium pentoxide film at plural stages, a niobium pentoxide film 10 nm in film thickness was formed on polycrystalline silicon at one stage, two stages or three stages and the leakage current densities thereof were compared. The niobium pentoxide film was formed by CVD using, as source gases, pentaethoxy niobium, and oxygen. At this time, the substrate temperature was set to 430xc2x0 C. Heat treatment was conducted at 600xc2x0 C. in oxygen gas flow for one minute. FIG. 15 shows the effect of decreasing the leakage current density by the formation at the plural stages. The transverse axis thereof represents voltage, and the vertical axis thereof represents the leakage current density. In the case of the single stage formation, a niobium pentoxide film was formed to have a thickness of 10 nm, and then heat-treated. In the case of the two-stage formation, a niobium pentoxide film was formed to have a thickness of 5 nm, and then heat-treated. Thereafter, a niobium pentoxide film was formed to have a thickness of 5 nm, and again heat-treated. In the case of the three-stage formation, a niobium pentoxide film was formed to have a thickness of 3 nm, and then heat-treated. Thereafter, a niobium pentoxide film was formed to have a thickness of 3 nm, and again heat-treated. Furthermore, a niobium pentoxide film was formed to have a thickness of 4 nm, and then heat-treated. As is evident from FIG. 15, by forming the niobium pentoxide film at the plural stages, the leakage current is decreased. One of reasons for this fact is that: by making the film thickness per layer of the layered film small by the plural-stage formation, it is possible to solve the problem that when the heat treatment temperature is made lower, oxygen does not diffuse easily in the film so that the efficiency of removing carbon, which should be discharged as carbon dioxide, deteriorates.
In order to obtain this effect of decreasing the leakage current, it is desired to make all of the layers thin. However, this effect can be obtained by making thin the thickness of any one of the plural insulator layers made of niobium pentoxide in the capacitor. This is because even when only one of the insulator layers is improved in leakage current property, the leakage current property of the whole is improved. Even in the case of a layered film composed of a tantalum pentoxide film and a niobium pentoxide film, as described as the first aspect, or even in the case of (Ta1xe2x88x92xNbx)2O5/(Ta1xe2x88x92yNby)2O5 satisfying the relationship of x less than y, the carbon-removing efficiency can be raised by making the film thickness thin, so that the leakage current property of the whole of the layered film can be improved.
The utilization of the above-mentioned aspect of the present invention makes it possible to realize a capacitor exhibiting a high dielectric constant and a small leakage current even by heat treatment at a low temperature of 700xc2x0 C. or less.
In the present context, examples wherein a layered film composed of a niobium pentoxide layer and another niobium pentoxide film is used have been described. However, the present invention is not limited to the example. In the case that a solid solution of tantalum pentoxide and niobium pentoxide is used as the material having a low crystallization temperature, a capacitor having the above-mentioned property can also be realized.
The following will compare and investigate the effects of the manner of forming a lamination of a niobium pentoxide film and a tantalum pentoxide film, which is the first aspect of the present invention, and the manner of forming a lamination of one niobium pentoxide film and another niobium pentoxide, which is the second aspect of the present invention.
According to the first and second aspects, it is possible to lower the crystallization temperature, prevent the bottom electrode and the barrier metal from being oxidized, and decrease the leakage current. However, the first aspect is superior to the second aspect in the decrease in the leakage current since the tantalum pentoxide film causing a smaller leakage current is used as the dielectric film. The second aspect is superior to the first aspect in easiness of production of a semiconductor device and a decrease in costs for the following reason: The respective dielectric films constituting the lamination are made of the same material; therefore, it is unnecessary to set separately a means for supplying a source gas for forming the niobium pentoxide film and a means for supplying a source gas for forming the tantalum pentoxide film, or handle the two different source gases for the layered film.
Subject matters common to the first and second aspects of the present invention are as follows: The dielectric film of a capacitor is made of a bi- or multi-layered insulator, thereby separating the crystal boundaries therein; and the lower dielectric layer of the capacitor is made into a layer comprising niobium pentoxide and the upper dielectric layer is made into a tantalum pentoxide layer, a niobium pentoxide layer, or a layer made of a composition of tantalum pentoxide and niobium pentoxide, whereby the crystallization temperature of the dielectric film can be made lower than that of a tantalum pentoxide film. As a result, the bottom electrode and the barrier metal of the capacitor can be prevented from being oxidized so that the capacitor can be formed as a capacitor having a good leakage current property.
An example of this capacitor includes a capacitor having a dielectric film composed of a lower layer made of a composition of tantalum pentoxide and niobium pentoxide and an upper layer made of niobium pentoxide. In the case of this capacitor, the crystallization temperature of the material used for the lower layer is higher than that of the material used for the upper layer. Thus, the capacitor does not have the effect of lowering the heat treating temperature of the upper layer to that of the lower layer by forming the dielectric film made of the material which originally has a high crystallization temperature on the dielectric film made of the material having a low crystallization temperature. However, the capacitor has an effect of making the heat treatment temperature for crystallizing the dielectric film lower than the heat treatment temperature for crystallizing any conventional tantalum pentoxide film. As a result, the bottom electrode and the barrier metal can be prevented from being oxidized by the heat treatment.
According to the present invention, the heat treatment temperature of the capacitor dielectric film can be made low; therefore, it is possible to suppress a decrease in the capacitance based on the oxidization of the polycrystalline silicon bottom electrode (MIS structure), and an increase in the contact resistance based on the oxidization of the barrier metal (MIM structure). In other words, it is possible to realize high integration based on making semiconductor capacitor elements fine; an improvement in the yield of semiconductor devices based on making the production process simple and more reliable; and so on. This makes it possible to increase signal quantity to improve the reliability of device operation, or decrease the height of the capacitor to decrease process load.
By making the capacitor dielectric film into a layered film and separating crystal boundaries in the dielectric film, leakage current can be decreased. Furthermore, by making the film thickness of the insulator film constituting the layered film small, stress in the film is reduced and the density of the film and the morphology thereof are improved so that leakage current can be decreased.